Sarah, left, and Melissa Adams show off the first same-sex marriage license approved at the Whatcom County Auditor's Office at the Whatcom County Courthouse in Bellingham, Wash., on Thursday, on the first day the state's law legalizing gay marriage goes into effect.

Washington — The US Supreme Court has set the stage for the most important judicial examination of gay rights in America – so far.

By agreeing on Friday to decide two different appeals involving same-sex marriage, the high court has set itself up as ultimate arbiter in an emotional and divisive battle at the heart of the nation’s raging culture war.

But the central question of these appeals is not whether the US Constitution protects a right to same-sex marriage.

However, should a majority of justices agree, the court’s decision might well provide a gay rights landmark equivalent to Brown v. Board of Education.

Supporters of the anti-gay marriage measures say they are merely upholding the traditional definition of marriage, as a union between one man and one woman.

Nonetheless, since 2004, more than 100,000 same-sex marriages have been performed in the US.

At its most basic, the issue the justices must determine is how equal protection is to be enforced in cases involving sexual orientation and marriage.

They have agreed to answer that question in the context of California’s Prop. 8 ban on same-sex marriage and the continued enforcement of DOMA, which bars gay and lesbian married couples from receiving the same federal benefits as heterosexual married couples.

The cases, Hollingsworth v. Perry (12-144) and US v. Windsor (12-307), likely will be set for oral argument in March or April, with decisions announced by late June.

Both decisions significantly expanded gay rights, but there is no indication yet where Justice Kennedy, a potential decisive swing vote, may come down in the same-sex marriage cases.

Specifically, the court must decide what level of judicial scrutiny to apply to the challenged measures.

Will a majority of justices decide that gay and lesbian couples are entitled to a higher protected status as members of a distinct and powerless group long subjected to discrimination?

Or will the court conclude that same-sex couples are entitled to no special protection when challenging measures that seek to uphold a definition of marriage that has prevailed – largely unchallenged – for much of civilized history.

In addition, the court must decide whether the government has a justifiable reason to treat male-female married couples differently than married couples of the same sex.

Traditionally, marriage has been accepted as an institution vital to the propagation of the human race. The ideal environment for that project was seen as an intact family unit with both biological parents serving as role models for their children.

But fertility and parenting skills were never prerequisites for marriage. And experience shows that same-sex married couples are just as capable as opposite-sex couples of raising adopted children as their own in a stable and loving home.

The trump card for traditionalists in the past was the argument that homosexual conduct violated society’s shared concept of sexual morality.

Justice Kennedy’s 2003 decision in Lawrence v. Texas eliminated that argument while invalidating a Texas law that criminalized sodomy for homosexuals but not for heterosexuals.

In that 6-3 decision, Kennedy said moral disapproval of a particular practice was not a sufficient reason to uphold a law prohibiting that practice.

In a dissent, Justice Antonin Scalia said the decision would undermine a range of laws based on morality, including against bigamy, adult incest, bestiality, and obscenity. He said it would also undercut any defense of the traditional definition of marriage.

“Today’s opinion dismantles the structure of constitutional law that has permitted a distinction to be made between heterosexual and homosexual unions, insofar as formal recognition in marriage is concerned,” Justice Scalia wrote in 2003.

At issue now – nine years later – is a string of judicial decisions in Massachusetts, New York, Connecticut, Arizona, and California striking down laws that draw distinctions between heterosexual and homosexual unions. In the process the courts are establishing new legal protections for same-sex couples, as well as for gay men and lesbians in general.

The decisions are the fruit of a concerted campaign by gay rights advocates to denounce marriage restrictions and those who support the restrictions as bigoted and irrational.

Gay rights advocates maintain that the American public will inevitably embrace same-sex marriage, in the same way that Americans embraced interracial marriage in the years following the high court’s 1967 decision in a case called Loving v. Virginia.

In the 15 years prior to that landmark decision by Chief Justice Earl Warren, 14 states repealed their bans on interracial marriage. By the time the issue arrived at the high court in 1967, only Virginia and 15 other states still had similar laws on the books.

In the same-sex marriage issue, today 37 states have passed constitutional amendments or statutes restricting marriage to one man and one woman.

In contrast, nine states plus the District of Columbia recognize same-sex marriage. The recognition has come as a result of court rulings, state legislation, and by state-wide referenda.

There is evidence that that the tide of public opinion may be shifting.

Prior to the November votes, ballot initiatives supporting same-sex marriage had been defeated 30 times in a row.

In addition, national public opinion polls now show a majority of Americans favor legalizing same-sex marriage, although significant opposition remains. And President Obama, who dodged the same-sex marriage issue during his first presidential campaign, endorsed gay marriage earlier this year, and last year his administration ended the Defense Department’s discriminatory “don’t ask, don’t tell” policy, which barred individuals who were openly gay or lesbian from serving in the US armed forces.

Defenders of the traditional concept of marriage say gay rights proponents are seeking to have courts dictate to the country how these issues should be resolved. They say the same-sex marriage issue should instead be left to voters and lawmakers to decide through channels of democratic government.

In the Prop. 8 case, the justices must decide whether a 2008 state-wide referendum in California violated the equal protection rights of gay and lesbian couples seeking to get married.

The referendum was a reaction to a 4-3 decision by the California Supreme Court that declared for the first time that the state’s constitution recognized a right of same-sex couples to marry.

An estimated 20,000 marriage licenses were issued to gay and lesbian couples in California in the five months following the decision. Meanwhile, supporters of the traditional definition of marriage went to work organizing a state-wide vote to amend the California constitution and restore the definition of marriage to a union between a man and a woman.

The referendum passed 52 percent to 48 percent, with more than seven million votes cast in favor of the amendment.

A group of same-sex couples challenged Prop. 8, arguing in federal court that the referendum violated their constitutional right to marry. A federal judge agreed, and declared Prop. 8 unconstitutional.

On appeal, a panel of the Ninth US Circuit Court of Appeals ruled 2 to 1 against Prop. 8. But rather than declaring whether the US Constitution protects a right to gay marriage, the appeals court instead said the state-wide ballot initiative violated the rights of gay and lesbian couples by taking away their opportunity to marry after it had been granted by the State Supreme Court.

The federal appeals court said the referendum was designed to punish same-sex couples because of their homosexuality and thus violated their constitutional right to be treated in the same say as heterosexual couples.

In addition to the California Prop. 8 case, the US Supreme Court justices agreed to examine a challenge by an elderly woman in New York City to the constitutionality of DOMA.

DOMA bars same-sex married spouses from receiving the same federal benefits that are readily available to heterosexual married couples. There are more than 1,100 such benefits, including spousal coverage on health insurance plans, social security survivor payments, and the ability to file a joint federal tax return.

The suit, filed on behalf of Edith Windsor, charges that Congress had no valid reason to deny equal federal benefits to similarly situated same-sex spouses whose marriages are just as legal and recognized as male-female marriages in same-sex marriage states.

Specifically at issue in her case is a $363,000 tax bill the federal government says she owes in estate taxes following the death of her life-long partner, Thea Spyer.

If the married couple had been a man and a woman, the government would have honored the usual spousal waiver. But since both spouses were women, DOMA required that Ms. Windsor be treated as if she was single and had never been married.

Among reasons offered to justify DOMA: to create an incentive for responsible procreation and child-rearing within a marriage, to uphold the traditional definition of marriage and its meaning in a variety of other federal laws, to foster uniform eligibility for federal benefits regardless of shifting state marriage laws, and to conserve federal funds.

Government lawyers successfully defended the statute during the Bush administration, but in 2011, President Obama and Attorney General Eric Holder decided that the law was unconstitutional. They announced that the administration would continue to enforce DOMA’s provisions (by denying benefits to same-sex married couples), but would refrain from defending DOMA’s constitutionality in court.